What Are the Reasons for the Decrease in Electrical Resistivity of EDI Equipment?

During the operation of EDI ultra-Pure Water equipment, if there is a decrease in water production resistivity, it is usually closely related to influent water quality, operation parameter control, pollutant accumulation, and other factors. The following is a combination of specific data and case analysis, explaining the main factors leading to resistivity anomaly and coping strategies.

1. Impact of substandard water quality of Reverse Osmosis pretreatment
As the front-end processing unit of EDI, the effluent quality of the reverse osmosis system directly determines the water production performance of EDI. If the conductivity, hardness, or variable metal of Ro Water exceeds the standard, the desalting efficiency of the EDI module will be significantly reduced.
The challenge of high salinity raw water: When the salt content of raw water exceeds 500mg/L, the desalting rate of a single-stage RO may be less than 98%, resulting in a water conductivity of more than 10μS/cm. At this time, the two-stage RO series process can stably control the conductivity at 1~3μS/cm. For example, after an electronics company replaced a single-stage RO with a two-stage RO, the inlet salt load of EDI was reduced by 70% and the resistivity was increased to more than 15MΩ·cm.
Synergistic effect of CO₂ with pH: When the influent CO₂ concentration exceeds 20ppm, the content of HCO₃⁻ in water is increased, which dissociates into H + and CO ²⁻ in EDI electric fields, increasing conductivity. In one case, after the degassing membrane was installed, the CO₂ concentration dropped from 35ppm to 5ppm, and the EDI water production resistivity recovered from 12 mω ·cm to 17 mω ·cm. At the same time, adding drugs to adjust the influent pH to 7.5-8.0 (weak alkaline environment), can inhibit CO₂ dissolution, and reduce ion migration interference.

2. Interference of out-of-control current parameters on ion migration
The operating current of the EDI system should be dynamically adjusted according to the water quality. Either too high or too low current will destroy the balance between ion exchange and regeneration.
Negative effects of current overload: Experiments have shown that when the current density exceeds 40mA/cm², there is an excessive build-up of H + and OH⁻ ions from hydro-ionization, leading to an increased risk of scale formation in concentrated water chambers. In the case of a photovoltaic company, after adjusting the current from 2.5A to 2.0A, the concentrated water conductivity decreased from 150μS/cm to 80μS/cm, and the water resistivity increased by 18%.
Polarization effect and reverse diffusion: If the current exceeds the load limit of the membrane pile (usually 120% of the rated value), the polarization phenomenon will cause reverse diffusion. For example, one laboratory test shows that when an electric current exceeds the limit by 10%, the rate of inverse diffusion of Na + and the amount of Cl + ⁻ increase by 2.5 times, resulting in a sharp drop in the resistivity of water production from 18 mω ·cm to 13 mω ·cm.

3. Irreversible damage of iron pollution to membrane pile
Iron compounds are high-risk pollutants in EDI systems, and their sources include pipeline corrosion and excessive iron content in raw water.
The form and adsorption properties of iron: Fe² + readily oxidizes in water into Fe(OH)₃ colloid (particle size 0.1~1μm), whose surface charge is electrostatic adsorbed with the quaternion group of the negative resin. EDI failure analysis of a power plant showed that when the influent iron content exceeded 0.1 ppm, the iron adsorption capacity of negative resin reached 3.2mg/g, resulting in a 40% decrease in exchange capacity.
Cumulative effect of iron contamination: During long-term operation, iron compounds form a dense layer on the membrane surface. Scanning electron microscopy (SEM) observations showed that the iron content on the surface of the contaminated film could reach 1.5% (EDX analysis data), obstructing ion channels and increasing film resistance. In one case, after cyclic cleaning with 5% citric acid and 0.1% reducing agent, the pressure drop of the membrane reactor was reduced by 30%, and the water production resistivity rose to 16MΩ·cm.

4. Inhibition of ion exchange by organic pollution
Low molecular weight organic matter (<200Da) can penetrate the RO membrane into the EDI system and interact physically and chemically with the resin and membrane.
Organic matter adsorption mechanism: humic acid, surfactants, and other negatively charged organic matter are easy to combine with negative resin. Experimental data show that when the TOC of influent water exceeds 50 ppb, the working exchange capacity of negative resin decreases by 25%~30%. In the case of a pharmaceutical company, adding an activated carbon filter after RO reduced TOC from 80ppb to 15ppb, and the electrical resistivity of EDI water production increased by 22%.
Membrane surface pollution kinetics: organic matter forms a gel layer on the membrane surface, extending the ion migration path. The dynamic simulation results show that when the organic matter coverage of the membrane surface exceeds 30%, the Na + migration rate decreases by 45% and the water production resistivity drops below 12 MΩ·cm. With regular use of 0.1% NaOH+0.5% NaCl mixed solution cleaning, the membrane flux can recover more than 90%.
The essence of EDI resistivity decline is the destruction of ion balance or the increase of mass transfer resistance. In the actual operation and maintenance, it is necessary to combine online monitoring (such as conductivity, TOC, and iron content) and regular performance evaluation to optimize the pretreatment process, adjust the operating parameters, and implement chemical cleaning to ensure the long-term stable operation of the EDI system.